1. Field of the Invention
The present invention relates to an image heating apparatus of electromagnetic (magnetic) induction heating type which is mounted on a copying machine, printer, facsimile, or other image forming apparatus that forms an image by an electrophotographic process, electrostatic recording process, or magnetic recording process.
Examples of the image heating apparatus are an apparatus that fixes an unfixed image on recording material and an apparatus that heats an image fixed on recording material to increase the glossiness of the image.
2. Description of the Related Art
Some image forming apparatuses use toner powder as a developer. In the process of fixing (heating) an unfixed toner image formed and borne on recording material, such image forming apparatuses typically employ a method of nipping the recording material between an image heating member and a pressure member and heating the toner image so that the toner image is fixed onto the recording material by pressure. The image heating member and the pressure member are rotating members that are pressed against each other to form a nip portion. At least the image heating member is heated by a heating unit to a predetermined temperature. There are various types of heating units for heating an image heating member. A heating unit of electromagnetic induction heating type includes an excitation coil which is opposed to a conductive layer. The excitation coil generates a magnetic field and thereby causes magnetic fluxes in the conductive layer lying in the magnetic field. This produces eddy currents within the conductive layer to generate heat. The electromagnetic induction heating method can directly heat the image heating member, so that the image heating member can generate heat in an extremely short time.
Japanese Patent Application Laid-Open No. 2000-075699 discusses a fixing apparatus that includes a belt member, a belt guide member, a pressure roller, and an electromagnetic induction heating apparatus. The belt member is supported without tension. The belt guide member is arranged close to an inner circumferential surface of the belt member. The pressure roller is pressed against the belt member. The electromagnetic induction heating apparatus heats the belt member. The belt member corresponds to a fixing member. The pressure roller corresponds to a pressure member. A thermistor, a temperature detection unit, is arranged in contact with the inner circumferential surface of the belt member, at the rotationally downstream side of a portion where the pressure roller is pressed against.
According to Japanese Patent Application Laid-Open No. 2000-075699, the thermistor is not configured to detect the temperature of the portion opposed to a coil, i.e., the temperature of a high temperature area where heat is generated. This gives rise to a problem of slow response when the belt temperature increases abnormally. To improve response to such situations, it is desirable to detect the temperature of the heat-generating portion, i.e., the portion opposed to a coil. A temperature detection member is thus desirably arranged on the inner circumferential surface of the belt member where opposite to a coil.
On the other hand, if an image heating member is thin as a belt which is an example of the image heating member, the skin depth can exceed the thickness of a conductive layer in the belt. In such a case, magnetic fluxes leak to the inner surface of the belt. With divergent leakage fluxes, the efficiency of heat generation drops due to less concentration of magnetic fluxes on the belt. Japanese Patent Application Laid-Open No. 2000-075700 discusses a configuration in which a magnetic core is arranged inside a belt, and a thermistor or a temperature sensing unit is arranged between the magnetic core and the belt.
For improved heat generation efficiency, the distance between the image heating member and the magnetic core needs to be reduced. An Electrical wire of a temperature detection member may be laid between the image heating member and the magnetic core and lead out from inside the image heating member.
With Such a configuration, however, the electrical wire and the image heating member are prone to come into contact with each other. The image heating member rotates. If the electrical wires frequently come into contact with the image heating member, the electrical wire and the image heating member wear out easily, failing to provide long life to each of the members.
A through portion may be formed through a part of the magnetic core, and the electrical wire may be laid in an internal space of the magnetic core via the through portion. Such a configuration can reduce the frequency of contact between the electrical wire and the image heating member.
In some case, the Electrical wire may be obliquely passed through a magnetic core. If a through portion is formed across the entire area where the electrical wires pass as illustrated in FIG. 7, the large opening of the through portion on the image heating member side causes irregularities of magnetic fluxes. Avoiding the irregularities limits the location of the through portion, such as to where the effect of heat generation from the image heating member is small. This decreases the degree of freedom in design.
A through portion that can reduce flux irregularities is thus desired to avoid the limitation on the layout of the through portion.